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ADVISORY - ACTIVE SOIL GAS INVESTIGATIONS Vapor probes are typically constructed of 1/8 inch or 1/4 inch diameter tubing, with a permeable probe tip. Alternate sub-slab sampling devices are commercially available. The reviewing agency should be consulted on the vapor probe construction prior to development of the workplan. Depending on the type of sub-slab probe construction, install a sand pack to minimize disruption of airflow to the sampling tip. Place the probe tip midway in the sand pack, as shown on Figure 2. Dry granular bentonite should be used to fill the borehole annular space to above the base of the concrete foundation. Hydrated bentonite should then be placed above the dry granular bentonite. The bentonite for this portion of probe construction should be hydrated at the surface to ensure proper sealing. Care should be used in placement of the bentonite to prevent post-emplacement expansion which might compromise both the probe and cement seal. The remainder of the hole should be filled with neat cement mixed with bentonite if the probe is permanent or with hydrated bentonite if the probe will be used for less than a year. Prior to the introduction of this material, the concrete surfaces in the borehole should be cleaned with a damp towel to increase the potential of a good seal and, ideally, all concrete dust should be removed from the borehole. All water used in the construction of the probe should be deionized, the cement should be contaminant-free and quick drying, and all metal probe components should be thoroughly cleaned to remove manufacturer-applied cutting oils. Each probe should be constructed with a gas-tight fitting and flush-mounted well box so that the probe completion is not a tripping hazard. Prior to sampling, at least two hours of time should elapse following installation of a probe to allow the construction materials to cure and allow for the subsurface to equilibrate (USEPA, 2006). The collection of sub-slab samples should follow the procedures in this Advisory, which recommends leak testing and shut-in testing. Purging and sampling rates should not exceed 200 milliliters per minute. To avoid air breakthrough from nearby, unobserved foundation cracks within the slab, DTSC recommends using sampling containers with volumes of less than or equal to one liter. If using passivated metal canisters, the canisters should be returned to the laboratory slightly depressurized, such as -2 inches of mercury. All laboratory analytical methods should meet the site-specific DQ0s and the analytical method reporting limits should be low enough for risk determination. July 2015 16